A+X is a new title that will debut after Avengers vs X-Men. It is part of the Marvel NOW (non) reboot.
To quote Marvel:
Coming later this year, A+X will showcase an array of Marvel’s top characters, including Wolverine, The Hulk, Captain America and Cable to start. On the creative side, Jeph Loeb, Dan Slott, Dale Keown and Ron Garney will be among early contributors, per Marvel Senior Editor Nick Lowe.
More on Marvel.com: http://marvel.com/news/story/19030/sdcc_2012_ax#ixzz20Yqku1kk
I think this sounds like a new version of Marvel Team-Up. One Avenger, and one X-Man in every issue. Teaming up to get over their differences and learn to work together.